Process of making cellulose esters of high uniformity



mats May" 7, 1929.

' UNITED STATES-PATENT oFFlcE.

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' This invention relates to processes ofmaking cellulose esters of highuniformity. One

object of the invention is to mix the cellulo sic material and acylatingliquid so thoroughly organic acidsitis highly desirable to have.

the acylation uniform throughout the fibers in order to-get a uniformlysoluble product. For example, in making cellulose acetate it isdesirable to get uniform acetylation in order to prevent the presence ofparticles in the product which are insoluble or partiall insoluble inthe customarysolvents. Suc particles impair the transparency of films,for instance. It has been customary to mix the cellulose in an excess ofthe acylating liquid under at least atmospheric pressure. Often the. ex-2 cess is allowed to remain during the esterification. .-Where only-arelatively small pro-. portion of such'l-iquidis to remain for thereaction, the excess is pressed out and removed. There is a tendency fornon-umform mixing under these. conditions. Eachfiber of celluloseis notfully wetted with the organic liquid and a non-uniform product results.I have found tha't'air trapped among the fibers or clinging to thelatter in theform of small bubbles isa cause of the trouble,'and thatthe latter. can be avoided or greatly lessened by putting the celluloseunder a vacuum, and regnating it'with the or- 40 ganic acylatinglhquidwhile it, isunder such vacuum. A. 'ce ulose ester of great uniformity isobtained-from the subsequent acylation at normal pressure.

-My invention is,a plicable to any of the cellulosic fibers use esters.'Since cellulose acetate is at present of major importance, I shalldescribe, y way of example, the use of acetylating liquids in carryingout my process, but it will be under: stood that my invention isapplicable to any similar organic acylating liquids.

The degree of vacuum employed can be varied considerably because somekinds of cellulose are less troublesome than others. The greater thetendency to retain air, the

'metho xtratlon, 100 parts by weight of clean cotton 7 portions. inmaking cellulose Application and "Au unt a, 1927. semi Ira-2131313.-

higher the vacuum employed- A. fairly strong vacuum f 14 to\ 40millimeters of mercury g vessa factory results in nearly all cases. j

In zgiplyinilililiy invention to the dope of, ma g. cellulose acetate,for illusare placed under a vacuum'of 16 mm. in. any suitableknownvacuum apparatus, the inner walls of which are resistant to theacetylatmg. bath. While the-vacuum is main- 'tained, thereis run intothe cotton and thoroughly mixed with it an acetylating bath containing.500 parts by weight of glacial acetic acid, 300 parts of aceticanhydrid, and

the catalyst, such as 2' parts of magnesium perchlorate trihydrate, orthe usual quantities of sulfuricacid, or zinc chlorid. The vacuum iskept up until air bubbles substantially cease to risefromthe mixture. Atroom temperature, this ordinarily does not take over an hour. Duringthis time, instead of a-continuous vacuum, the latter may be repeatedlyraised and lowered or even broken entirely and then renewed. ,The vacuumis finally stopped and the esterifying reaction then conducted atsubstantially atmospheric pressure and under the usual or normalconditions.

My present invention'is es ially useful for mixing the ingredients m theprocess disclosed in the application of the present inventor and CyrilJ. Stand, Serial No.

' 196,999, filed June 6th, 1927, for process of I making chloroforinsoluble cellulose acetate. In that process the proportion of acetylatingliquid to the cellulosic solids is ve small and the cellulose acetatedoes not dissolve. Uniformity .of impregnation of the cellulose with theacetic anhydrid is especiall im ortant to insure that all parts of t efi ers shall be converted into chloroform-soluble celluloseacetatewithout leaving insoluble My present vacuum treatment greatlyfacilitatesthe obtaining of this uni- 100 formity. For example, parts byweight of filter cotton (a highly purified dewaxed cotton). are placedin a vacuum chamber,

the walls ofwhich are acid resistant. A vacuum corresponding to between14 to 40 millimeters of mercury, say 16 mm., is created in the chamber.While the vacuum is kept'up, 220 parts by weight of acetic. anhydrid(94% strength), containing 0.12 parts by weight of sulfuric acid (98%preferably mixed or agitated with it.

strength), and 0.12 parts of phosphoric acid (85% strength), is run intothe cotton anal vacuum is maintained for about an hour at roomtemperature. The vacuum during this period may be-increased anddecreased or even broken entirely and repeated, in order rial No.196,999.

to facilitate. the impregnation. After the latter is com lete, t evacuum is released and the-acety ation' is carried out at atmosphericpressure, as in said application, Se-

It will be seen that in these examples the cellulose fibers are mixeduniforml with just the right amount of liquid for t e subsequentreaction. The liquid, containing from 2 to 5 times as much aceticanhydrid .as cellulose, remains in admixture with the cellulose duringthe subsequent reaction.

No excess of liquid is required, and conse the reaction between saidliquid and material.

2. In the process of 'making cellulose as ters of organic acids thesteps. of subjecting the cellulose fibers to be esterified to a vacuum,mingling with said fibers, while a vacuum is maintained, the quantity ofacylating liquid which is to be present during the vreaction, breakingsaid vacuum when air bubbles have been substantially "removed from themixture, and thereafter conducting the esterifying reaction. 4

3. In the process of making cellulose acetate thesteps of subjecting areaction mixture of cellulosic fibers and acetylating liquid containingacetic anhydrid and a catalyst to a vacuum to remove entrapped air,breakingsaid'vacuum and then conducting the acetylation reaction 4. Inthe process of making celluloseacetate, the steps of subjecting thecellulosic fibers to be acetylated to avacuum, mingling with saidfibers, while a vacuum is maintained, the quantity of acetylatingliquid, including acetic anhydrid which is to' be present during theacetylatlon, breaking the vacuum when air bubbles substantially cease toevolve from the mixture and thereafter conducting the acetylationreaction.

5. In the process of making cellulose acetate, subjectlng 100 parts byweight of eel luosic fibers to a vacuum corresponding to between 14 and40 millimeters of mercury,

while a vacuum is maintained, breaking the vacuum when air bubblessubstantially cease to evolve from the mixture, and then conducting theacetylating reaction under a pressure at least atmospheric.

Signed at Rochester, New York, this 19th day of ,Aug.', 1927.

HARRY LE B, GRAY.

5 mixing acetylating liquid containin 200 to;

500 parts of acetic anhydrid with said fibers

